1. Field of the Invention
The present invention relates to a signal evaluation system and method.
2. Description of the Prior Art
Successful indoor reception of terrestrial video broadcasts is frequently challenging. In the case of analogue TV broadcasts, poor reception is characterised by a graceful degradation of image quality accompanied by familiar image artefacts such as ghosting and snowy pictures.
By contrast, the advent of digital terrestrial television (DTT) has enhanced received picture quality significantly as long as the received signal quality at the receiver input remains above a certain threshold. However below this threshold, picture quality rapidly diminishes or the picture disappears completely (referred to as ‘brick-wall reception performance’). Unfortunately, in the case of indoor reception with a set-top antenna, the reception conditions frequently encountered place the received signal quality below this performance threshold, resulting in a subjectively worse experience for the user than with analogue TV.
To address this problem, it is possible to use a so-called ‘smart antenna’. Such a smart antenna comprises an active (steerable) directional antenna that is able to receive configuration data, in order to adapt and preferably optimise antenna reception characteristics for each received TV channel as it is selected. To facilitate adoption of this technology, the US Consumer Electronics Association (CEA) developed a standard for the configuration data known as the CEA-909 standard.
The CEA-909 standard provides for a configuration signal from a receiver to a smart antenna for a given TV channel that comprises 2 bits for coarse direction specification, 2 bits for fine direction specification, 1 bit specifying horizontal or vertical polarisation, 2 bits to specify gain level, and 7 bits specifying the channel number.
FIG. 1 illustrates how four values N of the coarse direction and the four values M of the fine direction are combined [N, M] to provide 16 directions for the steerable smart antenna. It will be appreciated that the 16 directions and the four values of N and M are a non-limiting examples.
In order to initially select a suitable configuration for a given TV channel, it is necessary to evaluate a signal reception for some if not all possible permutations of configurations available. This can be a lengthy process. The evaluation is typically based upon a combination of received signal strength, and signal quality.
A signal strength of a received signal may be estimated for example from a signal to noise ratio, which may in turn for example be determined from a deviation of signals from a target symbol in a constellation of a modulation scheme in use (in a similar manner to modulation error ratio). Other indicators of the signal strength include a radio frequency (RF) signal strength or the gain value of an automatic gain control system.
Meanwhile, indicators of signal quality include metrics such as the bit error rate (BER) of the demodulated signal. Notably, a threshold bit error rate for acceptable viewing of a TV channel may be very low, requiring a considerable period of time to pass before a statistically significant estimate of BER can be obtained. In conjunction with a need to evaluate a large number of smart antenna permutations, this can undesirably prolong the process of configuring or re-configuring a smart antenna.
A combination of signal strength and/or signal quality indicators used by the designer of a digital receiver thus determines a responsiveness of a receiver and also a quality of the received signal, as a configuration of an antenna will be selected to maximise one or both of such indicators.
Consequently it is desirable to improve such indicators where possible to improve in turn both the responsiveness of the digital receiver when configuring the smart antenna, and the resulting quality of the received signal.